In recent years mass spectrometry has started to be used as a method to monitor the dynamics of protein folding. We will explore this aspect of MS using at least two different approaches: (i) The detection of protected protein domains by limited proteolytic isolation of protected species from SDS-PAGE gels and identification using further digestion and MALDI or ESI-MS. (ii) monitoring the kinetics of H/D exchange at physiological pH (fast exchange) followed by pepsin digestion at low pH (slow exchange) and identification of protected regions as a function of time. We will investigate the application of these techniques to biological problems, such as identification of different folds caused by mutations in the steroidogenic acute regulatory protein StAR.